Fingerprints are used to ensure identity, enabling secure access in many applications including, but not limited to, electronic devices such a mobile phones, tablet, computers, as well as home security systems, automobiles, secure buildings, laboratories and patient identification in medical settings.
Current fingerprint detectors are limited, with the well-known aspect being that most fingerprint detectors are spoofable by fake fingers—plastic fingers with ridges and valleys will fool even a fingerprint device that can distinguish depth. Finger print detecting performed by a variant of frustrated total internal reflection cannot distinguish between a plastic faux finger and a flesh and blood finger.
Sub-dermal fingerprints, where the skin beneath the dermal or surface layer is imaged and analyzed provide higher certainty as to identification. The sub-dermal fingerprint has the same pattern as the surface fingerprint of an individual. Sub-dermal scans can be made using optical coherence tomography (OCT), and an OCT depth scan provides a fingerprint scan that can assure the target finger is living tissue.
One optical technique to image fingerprints involves use of a prism a surface of which is illuminated in a manner that light is subject to total internal reflection (tir or TIR) when there is no skin contact with the prism. Where there is contact the total internal reflection is frustrated by the reduced refractive index mismatch, thereby enabling the capability of imaging the fingerprint with enhanced dependence on the presence of the ridges and valleys of real fingerprints.
Typically, prisms currently used in frustrated total internal reflection are thick—of a thickness that makes a TIR device undesirably thick (or deep) for use in small, light electronic devices such as, for example, smart phones and tablets.